Dog over-drinking water and the water is really foamy and thick? HELP.?

Okay so I've lately realized that my dog (4 year old German Shepherd) has been OVER-drinking water. He's constantly thirsty and I need to repeatedly refill his water bowl. He makes a MESS drinking. foamy thick saliva falls from his mouth and the water in his bowl turns almost a yellow color. The clear water suddenly turns thick, foamy and unusual. What's wrong with him?